Drought Monitor: Stormy weather pattern dominated lower 48

Weekly Weather Summary: An unsettled and stormy weather pattern dominated much of the lower 48 States, unfortunately highlighted by a deadly and destructive bout of tornadoes in the Nation’s midsection. The southwest Missouri city of Joplin took a direct hit on May 22 from an EF-5 twister, taking the lives of at least 120 people and injuring at least 750, with numerous people still missing. Two days later, additional tornadoes touched down in Oklahoma and Arkansas, killing at least a dozen people. This stormy weather pattern also dumped moderate to heavy rains across much of the northern half of the U.S. and on much of the Plains, greatly easing drought conditions in the south-central and central Plains. Unfortunately, little or no rain fell along the extreme southern tier of States, from southern Arizona into southern Texas and eastward along the Gulf and southern Atlantic Coasts. Temperatures averaged below-normal in the West, north-central Plains, and Southeast, and above-normal in the southern Plains, upper Midwest, Northeast, and Alaska.

Important Note: The drought depiction for south-central Louisiana has been changed this week to show the areas of drought in the Atchafalaya Basin. As a result, this does not take into account the areas of flooding that were released into this basin from the Morganza Spillway courtesy of the lower Mississippi River flooding. The actual flooded areas have been less extensive and slower to spread than initially forecast; however, the amount of flooding is still significant.

Upper Great Lakes Region: Light rains (0.5 to 1 inch) was reported in northeastern Minnesota and the UP of Michigan, with heavier totals just to the south of the D0 areas. Stream flows and soil moisture indices have shown some improvement recently, but this area still could use a good widespread soaking to eliminate the lingering long-term (12- to 18-months) dryness.

South Atlantic Seaboard and Eastern Gulf Coast States: Little or no rain fell on most of the drought-affected areas (southern and eastern Mississippi, most of Alabama, Georgia, and Florida, and eastern sections of the Carolinas) as moderate to heavy precipitation (1.5 to 4 inches) was recorded just to the north. There was enough rain, however, in central Virginia (0.75 to 1.25 inches) to trim away some of the western D0 edge as deficits have gradually been replaced with near-normal or small surpluses during the past 90-days. At Norfolk, VA, and Elizabeth City, NC, however, 90-day deficiencies were 4.08 and 3.64 inches, respectively, and D1(A) remained. Similar deficits existed on the Delmarva Peninsula of Maryland as D1 was added, while D1 was removed just to the south on Virginia’s Delmarva Peninsula as 2 inches of rain fell earlier in the week. Farther south, light rains (0.2 to 0.7 inches) maintained conditions in the central Carolinas, but an H Impact Line was added to delineate adequate short-term conditions from longer-term shortages in south-central North Carolina and north-central South Carolina.

In contrast, subnormal rainfall continued across the eastern Gulf and southern Atlantic States. The last week that this region received widespread, soaking precipitation was in late March and early April, and conditions continued to deteriorate. USGS stream flows at 1-, 7-, 14-, and 28-days continued to drop, and many in southern sections of Mississippi and Alabama, most of Georgia, northern Florida, and parts of the Carolinas were in the lower tenth percentile as of May 24. According to USDA/NASS state reports, topsoil moisture rated short or very short increased this week to 82, 75,54, 48, and 47 percent, in Georgia, Florida, Mississippi, South Carolina, and Alabama, respectively. Similarly, pasture and range lands rated poor or very poor rose to 41 and 47 percent in Florida and Georgia. In southern Florida, the combination of short-term dryness (90-days: 25 to 50 percent of normal rainfall) and long-term deficits (since October 1: Miami 8.83 inches; West Palm Beach 21.39 inches; Ft. Lauderdale 20.97 inches; Naples 11.52 inches) have greatly lowered river and lake levels and some wells. Lake Okeechobee stood at 10.41 feet (normal 13.19 feet) on May 22, below the critical line for water shortages. Accordingly, D0 through D3 was incrementally increased northward in Mississippi, Alabama, and into western Georgia and northern Florida, and in extreme southern Florida (D3 in Monroe and Dade counties) in response to the growing short-term deficiencies.

Lower Mississippi Valley: As mentioned in the above “Important Note”, the opening of the Morganza Spillway on May 15 (last opened in 1973) had projected to flood much of the Atchafalaya River basin in south-central Louisiana by now. But due to the basin’s hydrological complexity and uncertainty, especially in the southern end, the rate and extent of flooding was less than initially forecast. Where it has flooded, however, the flooding is still significant. In contrast, where the flood waters had not yet reached, U.S. Forestry officials reported that many marshes were dry with fish kills from lack of oxygen in the remaining pools of water in coastal parishes of southwest and south Louisiana. Therefore, due to the severity of the ongoing drought and the lack of a recent image of actual flooded land, the area of drought depiction was returned to all of Louisiana. And of course, the flooded regions of the Atchafalaya basin (not depicted) would be drought free.

Elsewhere across southern Louisiana, little or no rain fell once again. Light rains (0.5 to 1 inch) occurred in northeastern Louisiana and southeastern Arkansas, while 1.5 to 2.5 inches fell on western Louisiana and eastern Texas. Unfortunately, additional heavy rains (2 to 8 inches) inundated the already-saturated upper Delta region (western and northern Arkansas, southern Missouri), adding more water into the swollen main stems of the Mississippi River. The rainfall did provide some basis for slight drought reduction in eastern Texas, western Louisiana, and southern Arkansas as a 1-category improvement was made to areas with weekly totals of 2 inches or more. However, large long-term (12-months) deficits (15 to 20 inches) remained in portions of the lower Mississippi River Valley, and were between 25 to 35 inches at 18-months.

Central and Southern Plains: Numerous and widespread showers and thunderstorms, including many cells that became severe with tornadic activity (e.g. Joplin, MO), dropped copious amounts of rain (2 to 10 inches) on parts of northeastern Texas, most of Oklahoma (except the Panhandle), much of Kansas (except the southwest), Nebraska (and northward), and parts of Colorado. Needless to say, many areas of drought (D0 to D4) were improved by 1-category, and in some places, by 2-categories, especially where more than 4 inches fell.

In Texas, eastern sections were improved by 1-category where 1.5 inches or more rain was measured as the rains should improve forage grass growth and conditions. In northeastern Texas along the Red River, a 1-category improvement meant that some locations were finally out of drought (D-nothing). In east-central Texas (e.g. Robertson County), less improvement was made as hydrological conditions were worse than other areas to the north and east. In northern Texas, 2 to 3 inches of rain along the Oklahoma border eased drought by 1-category, but farther west near Childress, D4 remained as deficits were large, and rainfall totals were less. Where little or no rain fell (southern and western Texas), D4 was expanded into Brazoria, Bexar, and Wilson counties. USGS stream flows continued to be in the lower tenth percentile in southern and south-central Texas, but improving to near-normal in the northeast. According to USDA/NASS, Texas winter wheat was rated 76 percent poor or very poor, while similarly-rated pastures and ranges stood at 73 percent.

In Oklahoma and Kansas, one and even some two-category improvements were made where the amounts were the greatest. In Oklahoma and Kansas, a 1-category improvement was made to any D0-D3 area that received 1.5 inches or more, and a 2-category swing was made where rainfall exceeded 4 inches. In southwest Oklahoma where 2 to 6 inches fell, only a 1-category improvement to the D4 area was made as soils were so dry (down to at least 8 feet) that time is needed to see if the soil moisture recharge holds out. In Harmon County, the rains (1 to 2 inches) did little to moisten the soil profile except to the top 2 inches, and this area remained in D4. Where little or no rain fell on western sections of Oklahoma and Kansas, drought persisted, and in some instances increased. D4 was expanded northward into Ellis and Harper counties where no summer crops are expected following the failure of the winter wheat crop. In the Panhandle, D4 was increased in Cimarron and Texas Counties as less than 40 percent of normal precipitation has occurred over the past 180 days. USDA/NASS winter wheat and pasture conditions rated poor and very poor for Oklahoma were 80 and 42 percent, while Kansas was 55 and 36 percent.

In Nebraska and Colorado, additional moderate to heavy (1.5 to 4 inches) rains erased D0 across Nebraska except for a few small pockets where medium- and long-term shortages (at both 90-days and since October 1; less than 70 percent) lingered. This included extreme southeastern and south-central sections of the state. Farther west in Colorado, another wet week brought 2 to 3 inches of precipitation to northern and western portions of the state, providing a second week of additional improvements to the moisture conditions. A 1-category reduction in drought (D0 to D2) was made across northeastern Colorado where 2 to 3 inches fell, but with lower amounts to the south, D2 and D3 remained, especially in the southeast. A slight expansion of D2 into the San Luis Valley in south-central Colorado as winter precipitation was lower than surrounding regions. Even with the recent rains, USDA/NASS pasture and winter wheat conditions rated poor or very poor stood at 48 and 40 percent for Colorado.

The Southwest: Although little or no rain fell on southern sections of the Southwest (southern Arizona, southern and eastern New Mexico, west Texas, southeastern Colorado), unsettled weather did make it far enough south into the Four Corners Region to drop unseasonably heavy precipitation (0.5 to 2 inches) on parts of central Arizona, northwestern New Mexico, southwestern Colorado, and southeastern Utah). Flagstaff, AZ, was now at normal for the year with the latest round of precipitation. Even heavier amounts were reported north of this area. As a result, some slight improvements to D0 to D2 edges were made in central Arizona, D0 and D1 were trimmed in northwestern New Mexico, and D0 was removed from southwestern Colorado. In southwest Texas, another week of rain (0.5 inches) in western Crockett and northeastern Pecos counties slightly reduced D4 there.

Hawaii and Alaska: Scattered light to moderate (0.5 to 2 inches) showers fell on the eastern (Big Island) and western (Kauai and Oahu) Hawaiian Islands while little or no rain was reported in the central (Maui, Molokai, Lanai) islands. On Maui, abnormal dryness was expanded to cover the rest of the island as low irrigation ditches were reported in eastern sections, along with lower yields at the Upcounty Agriculture park due to lower irrigation flows. Reduced forage has also led to an encroachment of deer onto farms and pastures. In contrast, the Big Island saw some improvement with recent rains. The D2 area was split in half as the mid-slopes received good April rains, and the pastures recovered. Unfortunately, the vegetation did not recover in the lower elevations near Kawaihae and upper elevations near Pohakuloa as D2 remained. On the western Big Island lower Kona slopes, ample rains have eliminated lingering D0 as farmers in much better shape than last year. Lastly, the southern Big Island improved from D1 to D0 near Kapapala and Pahala area with continued shower activity. Elsewhere, conditions stayed the same.

In Alaska, light to moderate precipitation (0.3 to 1 inch) fell on portions of the south-central and southeastern coasts, and on west-central interior sections. Locally, 2 to 4 inches of rain was observed on Kodiak Island and on the eastern Kenai Peninsula, but amounts were not large enough to eliminate long-term deficits. In the short-term (30- to 90-days), precipitation in southwestern and southeastern Alaska is close to or above-normal, however, in the long-term (since September 1, 2010, the start of the cold season), much larger deficits have accumulated. This included departures of -10 to -30 inches since September 1 along the normally wetter coastal locations. Accordingly, the D0(H) area was slightly modified to depict the larger long-term deficits.

Looking Ahead: During the next 5 days (May 26-30), a cold front will gradually push eastward out of the Nation’s midsection and off the East Coast later this week, bringing showers and thunderstorms to the eastern third of the Nation. Meanwhile, a series of Pacific storm systems will impact the Northwest and track into the northern Plains and upper Midwest, dropping light to moderate precipitation across the northern third of the U.S. Unfortunately, the southern tier of States will see little or no precipitation, although the Southeast may receive some light rain from the frontal passage. Shower activity may increase over the weekend in southern Florida. Above-normal readings will gradually spread from the southern Plains and South and push northward into the Midwest and East, while the West remains cool. Highs near 100 deg F may occur in parts of the Southeast by May 31.

The 6-10 day CPC outlook (May 31-June 4) calls for enhanced odds of subnormal precipitation from the central and southern Rockies across the south-central Plains and eastward throughout most of the Southeast and Atlantic Coast States. Above-normal precipitation is expected in the Northwest and upper Midwest. Subnormal temperatures are forecast for the Far West while unseasonable warmth envelops the eastern half of the Nation. In Alaska, wetter and warmer weather is probable in the southwest while drier and cooler conditions are expected in the east.

Author: David Miskus, NOAA/NWS/NCEP/CPC

Dryness CategoriesD0 ... Abnormally Dry ... used for areas showing dryness but not yet in drought, or for areas recovering from drought.